Preheat your oven to 425°F (220°C).
Grease four ramekins with butter and dust lightly with cocoa powder.
In a microwave-safe bowl, melt butter and dark chocolate together until smooth, stirring until combined.
Whisk in powdered sugar until smooth.
Add the eggs and egg yolks, whisking until combined, followed by the vanilla extract.
Sift in the flour and gently fold until just combined.
Pour the batter into the prepared ramekins, filling them about 2/3 of the way full.
Bake for 12-14 minutes, or until the edges are firm but the center is still soft.
Let the cakes cool for 1 minute before inverting onto plates.
Serve immediately with optional accompaniments.